Atmospherical pressure is around 760 mmHg, while blood pressure is on average 100 mmHg. Then why do you bleed from cuts, and why does a cut aorta spray blood? The atmosphere should press the blood back in, or at least stop it from spraying.

2 Answers
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When they say blood pressure is 100 mmHg, that really means 100 mmHg higher than atmospheric pressure. It's a gauge pressure, not an absolute pressure. The corresponding absolute pressure would be about (760 + 100) mmHg.

$\begingroup$This is incorrect. See the other answer, for example. Liquids and gasses do not necessarily exert the same pressure. They can be at a wide range of pressures depending on other factors. They also weren't asking why blood flows; but why it is able to spray, which does partially relate to it's pressure.$\endgroup$
– JMacApr 24 at 14:49